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			WERE RETARDS INVENTED BY THE BIG BANG?
	{{dablink\|For other senses of this word, see ].}}

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	The term '''universe''' has a variety of meanings based on the context in which it is described. In ] ] terms, the universe is the summation of all particles that exist and the space in which all events occur which has an equivalent idea amongst some theoretical scientists known as the '''total universe'''. In ] terms, the universe is thought to be a ] or ] ] in which all matter and energy exist. (It has been hypothesized by some scientists that the universe may be part of a system of many other universes, known as the ].) The terms '''known universe''', ''']''', or '''visible universe''' are often used to describe the part of the universe that can be seen or otherwise observed by humanity. Due to the fact that ] removes vast parts of the total universe from our ], most cosmologists currently accept that it is impossible to observe the whole continuum and may use '''our universe''', referring only to that knowable by human beings in particular.

	== Expansion and age, and the Big Bang theory ==
	{{main\|Age of the universe}}
	{{cosmology}}
	The most important result of ], the understanding that the universe is expanding, is derived from ] observations and quantified by ]. Extrapolating this expansion back in time, one approaches a ], a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the ] ], the dominant model in cosmology today. The ] from the time of the Big Bang, was estimated to be about 13.7 ] (13.7 × 10<sup>9</sup>) years, with a ] of about 1 % (± 200 million years), according to ]'s ] (Wilkinson Microwave Anisotropy Probe). However, this is based on the assumption that the underlying model used for data analysis is correct. Other methods of estimating the age of the universe give different ages.

	A fundamental aspect of the Big Bang can be seen today in the observation that the farther away from us ] are, the faster they move away from us. It can also be seen in the ] which is the much-attenuated radiation that originated soon after the Big Bang. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an early period of ] following the Big Bang.

	=== Size of the universe and observable universe ===
	{{main\|Observable universe}}
	There is disagreement over whether the universe is indeed finite or infinite in spatial extent and volume.

	However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite ], is certainly finite. The edge of the ] is 13.7 billion ]s (4.19 ]) distant. The ''present distance'' (]) to the edge of the observable universe is larger, due to the ever increasing rate at which the universe has been expanding; it is estimated to be about 78 ] light years (7.8 × 10<sup>10</sup> light years, or 7.4 × 10<sup>26</sup> m). This would make the comoving ], of the known universe, equal to 1.9 × 10<sup>33</sup> cubic light years (assuming this region is perfectly ]). The observable universe contains about 7 × 10<sup>22</sup> ]s, organized in about 100 billion ], which themselves form ] and ]s. The number of galaxies may be even larger, based on the ] observed with the ]. The Hubble Space Telescope discovered galaxies such as ], which are over 13 billion ] from Earth.

	Both popular and professional research articles in cosmology often use the term "universe" when they really mean "observable universe." This is because unobservable physical phenomena are scientifically irrelevant; that is, they cannot affect any events that we can perceive. See also ].

	We live in the center of the universe that we observe, in apparent contradiction to the ] which says that the universe is more or less uniform and it has no distinguished center. This is simply because light does not travel infinitely fast, and we make observations of the past. As we look further and further away, we see things from epochs (times) closer and closer to the limit of time, which equals zero, according to the Big Bang model. Since light travels at the same speed in any direction towards us, it is reasonable to suggest that we live at the center of our observable universe.

	== Shape of the universe ==
	{{main articles\|], and ]}}
	An important open question of cosmology is the shape of the universe. Mathematically, which ] represents best the spatial part of the universe?

	Firstly, whether the universe is spatially ''flat'', i.e. whether the rules of ] are valid on the largest scales, is unknown. Currently, most cosmologists believe that the observable universe is very nearly spatially flat, with local wrinkles where massive objects distort ], just as a lake is (nearly) flat. This opinion was strengthened by the latest data from ], looking at "acoustic oscillations" in the cosmic microwave background radiation temperature variations.

	Secondly, whether the universe is ], is unknown. The universe has no spatial boundary according to the standard Big Bang model, but nevertheless may be spatially finite (]). This can be understood using a two-dimensional analogy: the surface of a ] has no edge, but nonetheless has a finite area. It is a two-dimensional surface with constant curvature in a third dimension. The ] is a three-dimensional equivalent in which all three dimensions are constantly curved in a fourth.

	If the universe is indeed spatially finite, as described, then traveling in a "straight" line, in any given direction, would theoretically cause one to eventually arrive back at the starting point.

	Strictly speaking, we should call the stars and galaxies "views" of stars and galaxies, since it is possible that the universe is multiply-connected and sufficiently small (and of an appropriate, perhaps complex, shape) that we can see once or several times around it in various, and perhaps all, directions. (Think of a ].) If so, the actual number of physically distinct stars and galaxies would be smaller than currently accounted. Although this possibility has not been ruled out, the results of the latest ] research make this appear very unlikely.

	== Fate of the universe ==
	Depending on the average density of matter and energy in the universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "]". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for the whole of ] (see ]). Other ideas of the fate of our universe include the ], the ], and ] theory. For a more detailed discussion of other theories, see the ].

	== Multiverse ==
	There is some speculation that multiple universes exist in a higher-level ] (also known as a megaverse), our universe being one of those universes. For example, matter that falls into a black hole in our universe could emerge as a Big Bang, starting another universe. However, all such ideas are currently untestable and cannot be regarded as anything more than speculation. The concept of parallel universes is understood only when related to ].

	== Other terms ==
	Different words have been used throughout ] to denote "all of space", including the equivalents and variants in various languages of "heavens," "cosmos," and "world." ] has also been used to this effect, although it is more specifically defined as a system that reflects in large scale one, some, or all of its component systems or parts. (Similarly, a ] is system that reflects in small scale a much larger system of which it is a part.)

	Although words like ] and its equivalents in other languages now almost always refer to the planet ], they previously referred to everything that exists—see ], for example—and still sometimes do (as in "the whole wide world"). Some languages use the word for "world" as part of the word for "outer space", e.g. in the ] word "Weltall".

	==See also==
	* ] - the color of the Universe
	* ]
	* ]
	* ]
	* ]

	== References ==
	* ] (1952). ''Relativity: The Special and the General Theory (Fifteenth Edition)'', ISBN 0-517-88441-0

	== External links ==
	* - a series of images at various scales, with explanations.
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	* by Jim Holt, on various arguments for and against an infinite Universe and parallel universes
	* by Max Tegmark
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	* by Exploreuniverse.com
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Revision as of 19:28, 8 March 2006

WERE RETARDS INVENTED BY THE BIG BANG?